The present invention relates to a discrimination circuit for discriminating whether or not the frequency of a signal is higher than a predetermined reference frequency.
For discriminating whether or not the frequency of a signal is higher than a predetermined frequency, there is a well-known circuit in which a direct current signal with a level corresponding to the frequency of the signal is produced and the level of the direct current signal is compared with a reference level corresponding to the reference frequency. However, this circuit requires a smoothing circuit for obtaining a direct current signal with a level corresponding to the frequency of the signal to be measured, and the response characteristics tend to be poor owing to the time constant of the smoothing circuit.
To remove this disadvantage of the prior art circuit, U.S. Pat. No. 4,463,269 discloses a frequency discrimination circuit employing two retriggerable monostable multivibrators. This circuit has a first retriggerable monostable multivibrator which is capable of producing a first pulse signal with a pulse width equal to the half of period of a reference frequency and a second retriggerable monostable multivibrator which is capable of producing a second pulse signal with a pulse width longer than the half of period of the signal to be measured, and is constructed in such a way that the first retriggerable monostable multivibrator is triggered by the signal to be measured and the second retriggerable monostable multivibrator is triggered by the output signal of the first retriggerable monostable multivibrator.
Therefore, in this prior art circuit, when the frequency of the signal to be measured is not higher than the reference frequency, the period of the output pulse signal from the first retriggerable monostable multivibrator is longer than that of the reference frequency, so that the second retriggerable monostable multivibrator continues to be triggered by the output pulse signal of the first retriggerable monostable multivibrator before the output level of the second retriggerable monostable multivibrator becomes low. As a result, the output level of the second retriggerable monostable multivibrator is continuously maintained at high level.
On the other hand, when the frequency of the signal to be measured is higher than the reference frequency, the output level of the first retriggerable monostable multivibrator is maintained at high level, so that the second retriggerable monostable multivibrator is not triggered. Thus, the output level of the second retriggerable monostable multivibrator is maintained at low level.
The prior art circuit mentioned above is able to discriminate whether or not the frequency is higher than a predetermined reference frequency with a response delay corresponding to the pulse width of the output pulse of the second retriggerable monostable multivibrator when the frequency of the signal to be measured is in the vicinity of the reference frequency. However, since it is a condition for the operation of the circuit that the width of the output pulse of the second retriggerable monostable multivibrator be longer than the period of the signal to be measured, the frequency discriminating operation becomes impossible in a case where the period of the signal to be measured becomes longer than the width of the output pulse of the second retriggerable monostable multivibrator due to a decrease in the frequency of the signal to be measured. If the pulse width is elongated, the response delay time will increase.
As a result, in the case where the frequency to be discriminated varies widely, as, for example, a signal indicative of the rotational speed of an internal combustion engine, the discrimination circuit will sometimes become inoperative for the reason mentioned above.